Hitherto, certain types of muramyldipeptide derivatives have been known to have useful immunotherapeutic activities. For example, U.S. Pat. No. 4,101,536 discloses muramyldipeptide derivatives and the salts thereof having potent immunoadjuvant activity and antitumor activity and being applicable as an agent for the immunotherapy of cancer for human and animals. These known compounds, however, have a free hydroxy group at the 1-position of muramic acid of N-acetylmuramyldipeptide and do not possess an S-substituted moiety at the 1-position.